In mobile communication systems such as those under Long Term Evolution (LTE), random access (RA) is used in processes for mobile communication terminals to initiate connections with base stations. Further, random access channels (RACH) are used as radio resources for random access. For example, a RACH can be set for 1 to 10 channels, on one radio frame.
Furthermore, 64 preambles can be allocated per one RACH. As the preamble, random access code that is either shared code or dedicated code is used. Shared code is code that is randomly selected when a terminal begins access. Dedicated code is, for example, code that is temporarily allocated by a base station, to a terminal performing handover.
Random access includes contention random access and non-contention random access. In contention random access, a terminal transmits shared code as a preamble when making random access. In non-contention random access, the terminal transmits dedicated code as a preamble when making random access. In non-contention random access, a terminal can be identified by the dedicated code and therefore, compared to contention random access, the completion of random access processing is faster.
Meanwhile, to reduce operation expenditure of wireless network infrastructure in LTE, standardization of Self Organizing Networks (SON) in LTE is advancing. An objective of SON is to optimize the random access process. In the optimization of the random access process, delay caused by the random access process is minimized.
To optimize the random access process, a technology is known that measures RACH quality and based on the measured results, controls random access parameters (see, for example, Japanese Laid-Open Patent Publication No. 2009-55356). In the technology recited in Japanese Laid-Open Patent Publication No. 2009-55356, according to the direction in which RACH quality changes, the increase or decrease of a parameter is determined. Random access parameters include allocated time, frequency, code or space; initial transmission power; increased transmission power range; maximum number of retransmissions, number of signatures; number of sub-channels; etc.
Nevertheless, in the conventional technology above, the number of dedicated codes and the number of shared codes used in random access preambles are not considered as random access parameters. Consequently, dedicated codes become insufficient and fallback from non-contention random access to contention random access occurs, and shared codes become insufficient increasing contention random access collisions. As a result, random access is not performed efficiently, arising in a problem of random access consuming time.
Further, in the technology recited in Japanese Laid-Open Patent Publication No. 2009-55356, since a random access parameter is repeatedly increased and decreased until the RACH quality becomes optimized, it takes time until the random access parameter optimally converges, arising in a problem of deteriorated random access quality.